ESTABLISHMENT OF THE DORSOVENTRAL PATTERN DURING EMBRYONIC-DEVELOPMENT OF DROSOPHILA-MELANOGASTER - A LOGICAL ANALYSIS

This report focuses on dorso-ventral patterning in the segmented regio n of the Drosophila melanogaster embryo. According to the concept of p ositional information, this pattern results from the different respons es of cells to the Dorsal-protein morphogen. This protein shows a dist ribution gradient along the dorso-ventral axis, with the highest conce ntration on the ventral side. Using the generalized logical formalism developed by R. Thomas and co-workers, the different cellular response s were analysed in terms of the intracellular loops between the regula tory genes. Two positive loops were found to be involved, each constit uting a switch which can be acted upon by the Dorsal morphogen to dete rmine the different cell types that make up the embryonic dorso-ventra l pattern. The novelty in this use of generalized logical formalism is the employment of a multilevel variable to represent a morphogen grad ient. The proposed model accounts for the essential qualitative effect s of the Dorsal gradient in the dorso-ventral determination process. T hree main conclusions may be drawn. Firstly, the gene twist needs to h ave two functional threshold concentrations, one for autoactivation an d the other for activation of the gene snail. Secondly, the autoactiva tion threshold must be smaller than that which activates snail. Thirdl y, the action of the gene snail on the maintenance function of the gen e twist is crucial for cells to be able to choose between the mesoderm or neuroectoderm developmental pathways. Furthermore, it is predicted that if the gene snail shows autoregulation, this will not be crucial for the determination of the embryonic D-V pattern. (C) 1997 Academic Press Limited.